1. Field of the Invention
The present invention relates to a process and apparatus for recovering mercury in an amalgam-type electrolytic process, and more particularly in such a process where mercury which is carried out through the hydrogen line in the form of vapor is recycled to the process.
2. Description of the Prior Art
Production of chlorine in alkali hydroxide by the amalgam process is well known, and typically sodium chloride is electrolyzed to produce chlorine and sodium hydroxide. In the first step of the process, brine is fed into one end of this cell along with mercury, and the mercury flows along the bottom of the cell and acts as a cathode. Graphite or more recently developed dimensionally stable anodes are provided so that the electrolysis current flows through the brine solution and forms chlorine gas which bubbles to the top of the cell and is removed typically at the end opposite the brine feed along with the used or lean brine. During the electrolysis, alkali metal such as sodium is formed in the mercury cathode and this forms a mercury amalgam which is also removed from the cell at the end opposite the mercury feed end.
In a separate stage, the mercury amalgam is moved to a decomposer, where it is reacted with water, typically in a countercurrent flow. The decomposer typically contains a graphite packing which provides paths for the flow of electrons. When sodium is the metal in mercury amalgam, sodium atoms leave the amalgam surface as sodium ions into the solution. The electrons left in the metal travel to the graphite surface where the electrons reduce the water to OH.sup.- ions and hydrogen. Mercury amalgam stripped of its sodium is recycled back to the cell. In a typical operation, the caustic and hydrogen are drawn off at the end of the cell where the mercury amalgam is fed in. The water-sodium reaction generates considerable heat, and the caustic and hydrogen removed at their exit end of the decomposer and generally quite hot. Thus, the hydrogen carried with it considerable water vapor and mercury vapor. This mercury vapor, which is swept out with the hydrogen, has been a source of a problem in the amalgam-cell systems for many years.
First of all, the mercury should be recovered to prevent losses of expensive mercury, and secondly, the mercury is poisonous and should not be disposed of into the general environment. Accordingly, considerable effort has been expended in the recovery of this mercury.
In a typical operation, the hydrogen gas from the decomposer is passed through a condenser where water and a major portion of mercury passing out with the hydrogen is collected. The mercury thus collected may be recycled to the cell, and the water rcovered may be recycled to the decomposer. In this way, considerable mercury is retained in the system. Formerly, in multicell units the mercury had to be redistributed to the individual cell according to losses therein. The hydrogen-gas stream passing through the condenser may be then further treated for removing mercury by such processes as that shown in U.S. Pat. No. 3,647,359, or any of the other techniques well known in the art.
Another system is suggested and shown in U.S. Pat. No. 2,588,469 wherein a hydrogen stream is passed downward through a hydrogen cooler, and water and mercury is condensed from the stream and then returned to the bottom end of the decomposer. This system has certain advantages, but it does not provide as efficient condensation as a good conventional condenser operating in its normal vertical pass. In addition, this patent does not disclose multicell production systems.
Another problem has arisen in connection with the conventional mercury-recovery procedures, where a multiplicity of cells and decomposers are utilized, in that the mercury recovered in the conventional condenser must be divided among the various cells and control of mercury distribution is quite difficult. In other words, a typical plant may have perhaps 10 to 50 cells with 10 to 50 companion decomposers, and the hydrogen stream from these decomposers is condensed to provide a rather large amount of mercury. This mercury then must be distributed back to each of the cells in accordance with the losses therefrom. It is this problem along with the general problem of mercury recovery to which the present invention is directed.